There are two dates commonly used for the version of record of a publication: online date and print date.
Wherever possible and available, both dates are added to the metadata in Dimensions and made available in the web app, via the API and Google BigQuery. As a result, some publications have a print date, some an online date, some both, and some publications have a date which is not identified as either if it's not clear from the data we've received.
In cases where there are two dates, the date that is used for sorting results by publication date and in the Publication Year filter is the earlier of the two. This is not always necessarily the online date, for example in cases where a publication was published in print a long time ago and has now been made available in digital format by the publisher.